Effects of Countersink Hole on Driving Torques of Screw in ...
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Research Article Kastamonu Uni., Orman Fakültesi Dergisi, 2019, 19 (2):259-265
Kastamonu Univ., Journal of Forestry Faculty
Doi:10.17475/kastorman.626376
259
Effects of Countersink Hole on Driving Torques of Screw in
Joints Constructed of Medium Density Fiberboard
Önder TOR
Kastamonu University, Forest Industry Engineering, Kastamonu, TURKEY
Received Date: 01.08.2019 Accepted Date: 20.09.2019
Abstract
Aim of study: The aim of this study was to investigate the effect of countersink along with some other
factors affecting screw driving torques in joints made of medium density fiberboard (MDF). There is limited
research has been done in the field of screw driving torques in wood based composites. In all of these
studies, the specimen consisted of a single wood-based composite material and metal plate which was used
for the consistency of the screw driving data. However, in this study, the screw driving torques were
obtained by the specimens consisted of two MDF testing blocks jointed by a screw.
Material and Method: In general, there were two main screw driving torques; seating torque (SET) and
stripping torque (STT). The MDF testing blocks had dimension of 150 mm long × 75 mm wide and two
different thicknesses were used. For upper testing block, 8-mm-thick MDF and for lower testing block, 18-
mm-thick MDF were used. Torques measurements were obtained by an adjustable torque screwdriver.
Factors were embedded screw orientation (face-to-face and face-to-side), pilot-hole diameter (2.5 and 3.0
mm), pilot-hole depth (12 and 16 mm) and countersink type (with and without countersink).
Main results: The results of statistical analysis indicated that the four-way interaction among the
embedded screw orientation, screw length, countersink type and pilot-hole diameter was significant on the
mean SET and STT in the MDF joints.
Highlights: This study will help MDF manufacturers to understand the screw performance of their
products in terms of screw driving torques.
Keywords: Pilot-hole Diameter and Depth, Embedded Screw Orientation, Torque Wrench, Countersink Hole
Orta Yoğunluklu Lif Levhalarda Havşa Deliğinin Vidalama
Torkları Üzerine Etkisi
Öz
Çalışmanın amacı: Bu çalışmada, orta yoğunluklu lif levhalarda (MDF) havşa deliği ve bazı faktörlerin
vidalama tork değerleri üzerine etkileri araştırılmıştır. Bu konu ile ilgili yapılan önceki çalışmalarda, ahşap
esaslı malzemelerde vidalama torklarının ölçülebilmesi için metal bir plaka ile kullanılmıştır. Ancak, bu
çalışmada vidalama tork değerleri direk olarak iki MDF numunesi alından ve yüzeyden olmak üzere
birleştirilerek elde edilmiştir.
Materyal ve Yöntem: Vidalama torkları incelendiğinde iki vidalama torku ön plana çıkmaktadır. Birisi
vidanın malzemeye tam olarak oturduğu anda ki vidalama torku (SET) diğeri ise vidanın malzeme
içerisinde boşta dönmesinden hemen önce ki maksimum tork (STT) olarak adlandırılır. MDF deney
örnekleri 150 mm uzunluğunda ve 75 mm eninde kesilmiştir. 8 ve 18 mm olarak iki farklı kalınlık
kullanılmıştır. Vidalama torkları ayarlanabilir tork anahtarı ile ölçülmüştür. Bu araştırmada ki faktörler şu
şekildedir; vidalama yönü (malzemenin yüz ve alın kısmı), kılavuz deliği çapı (2.5 ve 3.0 mm), kılavuz
deliği derinliği (12 ve 16 mm) ve havşa deliğinin durumu (havşalı ve havşasız).
Sonuçlar: İstatistiksel analiz sonuçlar, vidalama yönü, kılavuz deliği çapı ve derinliği ve havşa deliğinin
durumu arasında ki dörtlü etkileşimin anlamlı olduğunu göstermiştir.
Önemli Vurgular: Bu çalışma, MDF panel üreticilerinin malzemelerinde ki vidalama performansları
hakkında önemli bilgiler vermektedir.
Anahtar Kelimeler: Kılavuz Deliği ve Derinliği, Vidalama Yönü, Tork Anahtarı, Havşa Deliği
Citation (Atıf): Tor, Ö. (2019). Effects of Countersink Hole on Driving Torques of Screw in Joints Constructed of Medium Density Fiberboard. Kastamonu University Journal of Forestry Faculty, 19 (2), 259-265.
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
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Introduction
Connecting wooden pieces together has
been an issue for decades in the terms of
stiffness, strength, safety and integrity of the
wooden components in construction. The
driving screws into any type of a material
without a countersink which is defined as a
conical hole cut into a manufactured object
such as plastic, metal or wood. The
countersink allows the head of a countersunk
screw placed in the hole to sit flush with or
below the surface of the surrounding material.
In addition, the countersink removes the burr
left from a drilling or tapping operation
thereby improving the finish of the product
and removing any hazardous sharp edges.
Previous studies related to screw driving
torques in oriented strandboard (OSB) (Tor et
al., 2015) and PB (Tor et al. 2015; Yu et al.,
2015), medium density fiberboard (MDF)
(Tor, 2019) and wood plastic composite
(Kuang et al., 2017). In these studies, the flush
condition termed as SET and defined as that
in which screw head was fully seated the
surface of material whereas STT defined as
the maximum torque after passing the SET
and increasing the torque sharply. This is
because the formed threads being stripped by
the screws. Controlling the torque of screw
driven into any type of materials is a crucial
issue. If the applied torque is too little, the
screw possibly will move and slip in the
material. Diversely, applying too much torque
on turning screws will cause shearing off or
fracturing the formed threads by screw in the
material and stripping problems. Driving
torque requirements of screw are related to
screw length and diameter, panel density,
screw penetration depth (Carroll, 1972).
Didriksson et al. (1974) evaluated edge
splitting tendency of fiberboards by using
internal bond (IB) test. The results of the study
indicated that the edge splitting tendency of
fiberboards was decreased when the pilot-hole
diameter was increased from 60 to 85 % of the
major diameter of screw which cause low
screw holding capacity in the joints.
The objectives of this study were to 1)
obtain SET and STT values, 2) to investigate
the effects of countersink hole along with
pilot-hole diameter and depth, embedded
screw orientation in joints made of MDF, 3)
quantify the significant factors on the SET and
STT. The results from this study will help
MDF manufacturers to improve the strength
and stability of any structure made of their
MDF products in terms of fastening by
screws. Therefore, the boundary of torque
measurements between SET and STT should
carefully set to prevent issues with non-seated
and stripped screws. For this reason, the STT-
to-SET ratio can help to have a good margin
between the SET and STT. High STT-to-SET
ratio minimizes the potential for damage
caused by stripping whereas low ratio can be
acceptable only for skilled assembly
operations.
Materials and Methods
Materials and Specimen Preparation
A full-sized MDF panels received from a
factory in Kastamonu, Turkey, measured 2.44
m long × 1.22 m wide was used. All testing
blocks were kept in equilibrium moisture
content chamber at 20 ± 3°C and 65 ± 5
relative humidity for two weeks (ASTM D
4442-92, 2010). The experiments were
divided into two groups based on having a
countersink or not. In group #1, a shallow hole
drilled by a countersink drill bit into the face
of the upper MDF testing block at almost half
thickness to make the screw head flush
whereas there was no shallow hole drilled in
the testing blocks in group #2. In addition, the
pilot-hole diameters were drilled at the rest of
the thickness in the upper testing block. In the
case of lower MDF testing blocks, the pilot
hole-depths were 65% and 80% of the screw
length which made the pilot-hole depth 16 and
20 mm in the MDF joints. Both testing blocks
were drilled by 2.5 and 3.0 mm drill bits which
were 70 and 83% of the screw major diameter
(ASTM D1761-06 and D1037-06, 2010).
Experimental Design
In order to evaluate factors on SET and
STT in the MDF joints, a four-factor factorial
experiment with 15 replicates per
combination was conducted. These four
factors were embedded screw orientation
(face-to-face and face-to-side), pilot-hole
diameter (2.5 and 3.0 mm), pilot-hole depth
(16 and 20 mm) and countersink type (with
and without countersink). The face of a
specimen was the panel surface and named as
face-to-face oriented MDF joint. The side of
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the specimen was either edge parallel or end
parallel to panel machine direction since there
was no significant difference between edge-
and end-embedded screw orientations in
terms of screw driving torques (Tor et al.
2015) and named as face-to-side oriented
MDF joints (Figure 1c and 1d). Therefore, a
total of 480 tests on SET and STT was
performed on 240 MDF testing blocks. The
testing blocks had dimension of 150 mm long
× 75 mm wide and two different thicknesses
were used as 8 mm thick MDF for upper MDF
testing block and 18 mm thick lower MDF
testing block.
a) b)
c) d)
Figure 1. The upper MDF testing blocks with
a countersink (a) and without a countersink
(b); face-to-face (c) and face-to-side (d)
oriented MDF joints
Torque Measurements
The torque measurement apparatus
consisted of two different Kraftform
adjustable torque screwdrivers varied based
on the torque ranges (Wuppertal, Germany)
(Figure 2). The torque of controlled
screwdrivers ranged from 0.3 to 1.2 N.m and
1.2 to 3.0 N.m, respectively. They had
distinctly audible and noticeable excess-load
signal when the required torque was reached.
The measurement accuracy was ± 6%
accordance with the standard of ISO 6789-2.
a) b)
Figure 2. Kraftform adjustable torque
screwdrivers. The torque ranged from 0.3 to
1.2 N.m (a) and 1.2 to 3.0 N.m (b).
Results and Discussion
Mean SET and STT values and their
coefficient of variance values are given in
Table 1 which also shows the ratios of STT-
to-SET for each treatment combination. In
general, the ratio of STT-to-SET ranged from
2.0 to 2.6 for face-to-face oriented MDF joints
within both countersink types whereas the
ratio ranged from 1.7 to 2.2 for the face-to-
side oriented MDF joints. Overall, the ratios
less than 3, the operator needs to be careful
when driving screws in this type of joints
(Robert, 2012).
Mean SET and STT Comparisons
The STT had significantly higher values
than SET (Table 1). Thus, a general linear
model procedure for a four-factor balanced
analysis of variance (ANOVA) was
performed in order to analyze four main
effects and their interactions on means of SET
and STT of screws driven in MDF joints
separately. The ANOVA results indicated that
the four-factor interaction was statistically
significant on SET and STT at the 5%
significance level (Table 2). Thus, the
protected least significant difference (LSD)
multiple comparisons procedure was needed
to be performed to compare the mean
difference. A one-way classification of 16
combinations was created for both SET and
STT data sets to evaluate the mean differences
among the combinations using LSD values of
0.0362 N.m and 0.0601 N.m for data sets with
respect to the four interaction, respectively.
All outputs were provided by SAS software
2014 (SAS Institute Inc. Cary, NC, USA).
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Table 1. Mean values of SET and STT of driving screws into MDF joints.
Countersink
hole type
Screw
embedded
orientation
Pilot-hole
depth
(mm)
Pilot-hole
diameter
(mm)
Screw driving torques
(N.m)
Ratio
STT/SET
SET STT
C
Face-to-face
16 2.5 0.97 (8)* 2.07 (3) 2.1
3 0.83 (11) 1.71 (5) 2.3
20 2.5 0.85 (4) 1.85 (4) 2.2
3 0.63 (9) 1.68 (5) 2.6
Face-to-side
16 2.5 0.40 (10) 0.82 (11) 2.0
3 0.30 (4) 0.56 (13) 1.8
20 2.5 0.33 (10) 0.71 (13) 2.2
3 0.31 (7) 0.53 (13) 1.7
NC
Face-to-face
16 2.5 0.82 (4) 1.81 (5) 2.2
3 0.74 (10) 1.51 (13) 2.1
20 2.5 0.78 (12) 1.76 (2) 2.3
3 0.67 (7) 1.34 (5) 2.0
Face-to-side
16 2.5 0.42 (6) 0.86 (9) 2.0
3 0.35 (6) 0.61 (6) 1.7
20 2.5 0.31 (13) 0.63 (5) 2.0
3 0.30 (4) 0.49 (9) 1.8 *Values in parantheses are coefficient of variation (%).
Table 2. Summary of ANOVA results
Source SET STT
F-value p-value F-value p-value
Countersink hole type 6.9 <.0001 109.8 <.0001
Screw embedded orientation 4381 <.0001 9775.3 <.0001
Pilot-hole depth 262.79 <.0001 580.29 <.0001
Pilot-hole diameter 114.5 <.0001 126.61 <.0001
2-way interactions 0.1-47.82 0.749-<.0001 0.01-95.7 0.9078-<.0001
3-way interactions 2.17-17.35 0.0.142-<.0001 2.27-10.3 0.133-<.0001
4-way interaction 0.33 0.0021 15.25 0.0001
Pilot-hole Effects
In general, mean SET and STT values of
MDF joints drilled by the pilot-hole diameter
of 2.5 mm was higher than the corresponding
ones at 3.0 mm since less fibers were cut
through by a screw at the 3.0 mm-pilot-hole
diameter than the ones at the 2.5 mm. This
decrease could also be because of less friction
force between the surfaces of the screw itself
in the material (Yu et al. 2015). Even though
the pilot-hole diameter of 2.5 mm had higher
SET than the ones at the 3.0 mm, there was no
statistically significant difference among the
pilot-hole diameters in the case of driving
screw with the pilot-hole depth of 12 mm in
the face-to-side oriented MDF joints with and
without the countersink on the top. In all other
combinations, the pilot-hole diameter of 2.5
had higher mean SET and STT than the ones
at the 3.0 mm (Table 3). In general, the mean
SET and STT values when the pilot-hole
depth was 8 mm in the MDF joints was higher
than the corresponding ones at 12 mm (Table
4). Most of the cases, there was statistically
significant difference between the pilot-hole
depths of 8 and 12 mm where the 8-mm pilot-
hole depth had higher SET and STT than the
12 mm.
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Table 3. Mean comparisons of SET and STT in MDF joints for pilot-hole diameters within each
combination of pilot-hole depth, screw embedded orientation and countersink type.
Countersink
hole type
Screw embedded
orientation
Pilot-hole
depth (mm)
SET STT
Pilot-hole diameter (mm)
2.5 3 2.5 3
C
Face-to-face 16 0.97 A* 0.83 B 2.07 A 1.71 B
20 0.85 A 0.63 B 1.85 A 1.68 B
Face-to-side 16 0.40 A 0.30 B 0.82 A 0.56 B
20 0.33 A 0.31 A 0.71 A 0.53 B
NC
Face-to-face 16 0.82 A 0.74 B 1.81 A 1.51 B
20 0.78 A 0.67 B 1.76 A 1.34 B
Face-to-side 16 0.42 A 0.35 B 0.86 A 0.61 B
20 0.31 A 0.30 A 0.63 A 0.49 B * Means followed by a common letter are not significantly different at the 5% level.
Table 4. Mean comparisons of SET and STT in MDF joints for pilot-hole depth within each
combination of pilot-hole diameter, screw embedded orientation and countersink type.a
Countersink
hole type
Screw
embedded orientation
Pilot-hole
diameter (mm)
SET STT
Pilot-hole depth (mm)
16 20 16 20
C
Face-to-face 2.5 0.97 A 0.85 B 2.07 A 1.85 B
3.0 0.83 A 0.63 B 1.71 A 1.68 A
Face-to-side 2.5 0.40 A 0.33 B 0.82 A 0.71 B
3.0 0.30 A 0.31 A 0.56 A 0.53 A
NC
Face-to-face 2.5 0.82 A 0.78 B 1.81 A 1.76 A
3.0 0.74 A 0.67 B 1.51 A 1.34 B
Face-to-side 2.5 0.42 A 0.31 B 0.86 A 0.63 B
3.0 0.35 A 0.30 B 0.61 A 0.49 B * Means followed by a common letter are not significantly different at the 5% level.
Screw Embedded Orientation Effects
In general, the mean SET values ranged
from 0.63 to 0.97 N.m for the face-to-face
oriented MDF joint and from 0.30 to 0.42 N.m
for the face-to-side oriented MDF joint while
the STT mean values ranged from 1.34 to 2.07
N.m for the face-to-face orientation and from
0.49 to 0.82 for the face-to-side orientation
(Table 5). In the case of mean comparison of
screw embedded screw orientations, the mean
SET and STT values for the face-to-face
orientation was higher than the corresponding
ones for the face-to-side orientation in all
combinations of pilot-hole diameter, pilot-
hole depth and countersink type. This could be
explained by the density profile of the MDF
material. The MDF material used in this study
had three layers of different densities as
manufactured in the panel company. The
upper and lower surface layers had higher
densities which were about 0.85 g/cm3 at the 1
mm thickness of both layers while the core
density was about 0.50 g/cm3 at the 6 mm
thickness of middle layer. For the face-to-side
orientation, when the screw was driven into
MDF joints, the screw penetrated into the
middle layer of MDF material after passing
through the upper MDF face testing block.
This can lead to a decrease of SET and STT in
side orientation because of lower core density.
This can also lead to cause minor splits and
cracks which can lower STT values when the
pilot-hole diameter is less than 2.5 mm.
Therefore, the pilot-hole diameter needs to be
chosen very carefully when driving screw into
this kind of materials.
Countersink Hole Type Effects
In general, the mean SET and STT values
were higher when a countersink (C) was
drilled in middle of the upper MDF testing
block in the joints than the ones without the
countersink (NC) in all combinations except
one case (Table 6). The mean SET in the
face-to-face oriented MDF joint with no
countersink on the top with the 3.0 mm-pilot-
hole diameter and depth of 20 mm was
significantly higher than the corresponding
ones with the countersink on the top of upper
testing block.
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Table 5. Mean comparisons of SET and STT in MDF joints for screw embedded orientation within
each combination of pilot-hole diameter, pilot-hole depth, and countersink type.
Countersink
hole type
Pilot-hole
depth
(mm)
Pilot-hole
diameter
(mm)
SET STT
Screw embedded orientation
Face-to-face Face-to-side Face-to-face Face-to-side
C
16 2.5 0.97 A 0.40 B 2.07 A 0.82 B
3.0 0.83 A 0.30 B 1.71 A 0.56 B
20 2.5 0.85 A 0.33 B 1.85 A 0.71 B
3.0 0.63 A 0.31 B 1.68 A 0.53 B
NC
16 2.5 0.82 A 0.42 B 1.81 A 0.86 B
3.0 0.74 A 0.35 B 1.51 A 0.61 B
20 2.5 0.78 A 0.31 B 1.76 A 0.63 B
3.0 0.67 A 0.30 B 1.34 A 0.49 B *Means followed by a common letter are not significantly different at the 5% level.
Table 6. Mean comparisons of SET and STT in MDF joints for countersink type within each
combination of pilot-hole diameter, pilot-hole depth, and screw embedded orientation.
Screw
embedded
orientation
Pilot-hole
depth (mm)
Pilot-hole
diameter
(mm)
SET STT
Countersink hole type
C NC C NC
Face-to-face
16 2.5 0.97 A 0.82 B 2.07 A 1.81 A
3.0 0.83 A 0.74 B 1.71 A 1.51 B
20 2.5 0.85 A 0.78 B 1.85 A 1.76 B
3.0 0.63 B 0.67 A 1.68 A 1.34 B
Face-to-side
16 2.5 0.40 A 0.42 A 0.82 A 0.86 A
3.0 0.30 A 0.35 B 0.56 A 0.61 A
20 2.5 0.33 A 0.31 A 0.71 A 0.63 B
3.0 0.31 A 0.30 A 0.53 A 0.49 A *Means followed by a common letter are not significantly different at the 5% level.
Conclusion
In this study, the experimental results
indicated that embedded screw orientation,
screw length, countersink type and pilot-hole
diameter significantly affected the SET and
STT in the MDF joints. The pilot-hole
diameter of 2.5 mm had higher SET and STT
values than the one with 3.0 mm in all
combinations of countersink hole type,
embedded screw orientation, pilot-hole
diameter and depth. The MDF joints with the
countersink on the upper testing block had
higher SET and STT than the one with no
countersink. A possible explanation for this
was because of more friction force between
contacting surface of head of the screw and the
material occurred in the MDF joints and
ultimately more screw penetration attained in
the joint with the countersink. This study will
help MDF manufacturers to understand the
screw performance of their products in terms
of screw driving torques.
Acknowledgment
This study was founded by the Kastamonu
University, Kastamonu, Turkey (grant
number KUBAP01/2017-58).
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